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摘要：本文主要讲了Java中BlockingQueue的源码

一、BlockingQueue介绍与常用方法

BlockingQueue是一个阻塞队列。在高并发场景是用得非常多的，在线程池中。如果运行线程数目大于核心线程数目时，也会尝试把新加入的线程放到一个BlockingQueue中去。队列的特性就是先进先出很容易理解，在java里头它的实现类主要有下图的几种，其中最常用到的是ArrayBlockingQueue、LinkedBlockingQueue及SynchronousQueue这三种，这三个也是今天主要讲的类。

它主要的方法有

BlockingQueue的核心方法：
1、放入数据

（1） add(object)

队列没满的话，放入成功。否则抛出异常。

（2）offer(object):

表示如果可能的话,将object加到BlockingQueue里,即如果BlockingQueue可以容纳,则返回true,否则返回false.（本方法不阻塞当前执行方法的线程）
 （3）offer(E o, long timeout, TimeUnit unit)

可以设定等待的时间，如果在指定的时间内，还不能往队列中加入BlockingQueue，则返回失败。
（4）put(object)

把object加到BlockingQueue里,如果BlockQueue没有空间,则调用此方法的线程阻塞。直到BlockingQueue里面有空间再继续.
2、获取数据
（1）poll(time)

取走BlockingQueue里排在首位的对象,若不能立即取出,则可以等time参数规定的时间,取不到时返回null;
（2）poll(long timeout, TimeUnit unit)

从BlockingQueue取出一个队首的对象，如果在指定时间内，队列一旦有数据可取，则立即返回队列中的数据。否则知道时间超时还没有数据可取，返回失败。

（3）take()

取走BlockingQueue里排在首位的对象,若BlockingQueue为空,阻断进入等待状态直到BlockingQueue有新的数据被加入;
（4）drainTo()

一次性从BlockingQueue获取所有可用的数据对象（还可以指定获取数据的个数），通过该方法，可以提升获取数据效率；不需要多次分批加锁或释放锁。

二、ArrayBlockingQueue

一个基本数组的阻塞队列。可以设置列队的大小。

ArrayBlockingQueue的源码是比较简单的，下面是笔者抽取了一部分源码并加以注释。它的基本原理实际还是数组，只不过存、取、删时都要做队列是否满或空的判断。然后加锁访问。

package java.util.concurrent;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import java.util.AbstractQueue;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.lang.ref.WeakReference;
import java.util.Spliterators;
import java.util.Spliterator;

public class ArrayBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {

    private static final long serialVersionUID = -817911632652898426L;

    /** 真正存入数据的数组*/
    final Object[] items;

    /** take, poll, peek or remove的下一个索引 */
    int takeIndex;

    /** put, offer, or add的下一个索引 */
    int putIndex;

    /**队列中元素个数*/
    int count;

    /**可重入锁 */
    final ReentrantLock lock;

    /** 队列不为空的条件 */
    private final Condition notEmpty;

    /** 队列未满的条件 */
    private final Condition notFull;

    transient Itrs itrs = null;

    /**
     *当前元素个数-1
     */
    final int dec(int i) {
        return ((i == 0) ? items.length : i) - 1;
    }

    /**
     * 返回对应索引上的元素
     */
    @SuppressWarnings("unchecked")
    final E itemAt(int i) {
        return (E) items[i];
    }

    /**
     * 非空检查
     *
     * @param v the element
     */
    private static void checkNotNull(Object v) {
        if (v == null)
            throw new NullPointerException();
    }

    /**
     * 元素放入队列，注意调用这个方法时都要先加锁
     *
     */
    private void enqueue(E x) {
        final Object[] items = this.items;
        items[putIndex] = x;
        if (++putIndex == items.length)
            putIndex = 0;
        count++;//当前拥有元素个数加1
        notEmpty.signal();//有一个元素加入成功，那肯定队列不为空
    }

    /**
     * 元素出队，注意调用这个方法时都要先加锁
     *
     */
    private E dequeue() {
        final Object[] items = this.items;
        @SuppressWarnings("unchecked")
        E x = (E) items[takeIndex];
        items[takeIndex] = null;
        if (++takeIndex == items.length)
            takeIndex = 0;
        count--;/当前拥有元素个数减1
        if (itrs != null)
            itrs.elementDequeued();
        notFull.signal();//有一个元素取出成功，那肯定队列不满
        return x;
    }

    /**
     * 指定删除索引上的元素
     *
     */
    void removeAt(final int removeIndex) {
        final Object[] items = this.items;
        if (removeIndex == takeIndex) {
            items[takeIndex] = null;
            if (++takeIndex == items.length)
                takeIndex = 0;
            count--;
            if (itrs != null)
                itrs.elementDequeued();
        } else {
            final int putIndex = this.putIndex;
            for (int i = removeIndex;;) {
                int next = i + 1;
                if (next == items.length)
                    next = 0;
                if (next != putIndex) {
                    items[i] = items[next];
                    i = next;
                } else {
                    items[i] = null;
                    this.putIndex = i;
                    break;
                }
            }
            count--;
            if (itrs != null)
                itrs.removedAt(removeIndex);
        }
        notFull.signal();//有一个元素删除成功，那肯定队列不满
    }

    /**
     *
     * 构造函数，设置队列的初始容量
     */
    public ArrayBlockingQueue(int capacity) {
        this(capacity, false);
    }

    /**
     * 构造函数。capacity设置数组大小 ，fair设置是否为公平锁
     * capacity and the specified access policy.
     */
    public ArrayBlockingQueue(int capacity, boolean fair) {
        if (capacity <= 0)
            throw new IllegalArgumentException();
        this.items = new Object[capacity];
        lock = new ReentrantLock(fair);//是否为公平锁，如果是的话，那么先到的线程先获得锁对象。
        //否则，由操作系统调度由哪个线程获得锁，一般为false，性能会比较高
        notEmpty = lock.newCondition();
        notFull =  lock.newCondition();
    }

    /**
     *构造函数，带有初始内容的队列
     */
    public ArrayBlockingQueue(int capacity, boolean fair,
                              Collection<? extends E> c) {
        this(capacity, fair);

        final ReentrantLock lock = this.lock;
        lock.lock(); //要给数组设置内容，先上锁
        try {
            int i = 0;
            try {
                for (E e : c) {
                    checkNotNull(e);
                    items[i++] = e;//依次拷贝内容
                }
            } catch (ArrayIndexOutOfBoundsException ex) {
                throw new IllegalArgumentException();
            }
            count = i;
            putIndex = (i == capacity) ? 0 : i;//如果putIndex大于数组大小 ，那么从0重新开始
        } finally {
            lock.unlock();//最后一定要释放锁
        }
    }

    /**
     * 添加一个元素，其实super.add里面调用了offer方法
     */
    public boolean add(E e) {
        return super.add(e);
    }

    /**
     *加入成功返回true,否则返回false
     *
     */
    public boolean offer(E e) {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lock();//上锁
        try {
            if (count == items.length) //超过数组的容量
                return false;
            else {
                enqueue(e); //放入元素
                return true;
            }
        } finally {
            lock.unlock();
        }
    }

    /**
     * 如果队列已满的话，就会等待
     */
    public void put(E e) throws InterruptedException {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();//和lock()方法的区别是让它在阻塞时也可抛出异常跳出
        try {
            while (count == items.length)
                notFull.await(); //这里就是阻塞了，要注意。如果运行到这里，那么它会释放上面的锁，一直等到notify
            enqueue(e);
        } finally {
            lock.unlock();
        }
    }

    /**
     * 带有超时时间的插入方法，unit表示是按秒、分、时哪一种
     */
    public boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException {

        checkNotNull(e);
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == items.length) {
                if (nanos <= 0)
                    return false;
                nanos = notFull.awaitNanos(nanos);//带有超时等待的阻塞方法
            }
            enqueue(e);//入队
            return true;
        } finally {
            lock.unlock();
        }
    }

    //实现的方法，如果当前队列为空，返回null
    public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (count == 0) ? null : dequeue();
        } finally {
            lock.unlock();
        }
    }
     //实现的方法，如果当前队列为空，一直阻塞
    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == 0)
                notEmpty.await();//队列为空，阻塞方法
            return dequeue();
        } finally {
            lock.unlock();
        }
    }
    //带有超时时间的取元素方法，否则返回Null
    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == 0) {
                if (nanos <= 0)
                    return null;
                nanos = notEmpty.awaitNanos(nanos);//超时等待
            }
            return dequeue();//取得元素
        } finally {
            lock.unlock();
        }
    }
    //只是看一个队列最前面的元素，取出是不删除队列中的原来元素。队列为空时返回null
    public E peek() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return itemAt(takeIndex); // 队列为空时返回null
        } finally {
            lock.unlock();
        }
    }

    /**
     * 返回队列当前元素个数
     *
     */
    public int size() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return count;
        } finally {
            lock.unlock();
        }
    }

    /**
     * 返回当前队列再放入多少个元素就满队
     */
    public int remainingCapacity() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return items.length - count;
        } finally {
            lock.unlock();
        }
    }

    /**
     *  从队列中删除一个元素的方法。删除成功返回true,否则返回false
     */
    public boolean remove(Object o) {
        if (o == null) return false;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            if (count > 0) {
                final int putIndex = this.putIndex;
                int i = takeIndex;
                do {
                    if (o.equals(items[i])) {
                        removeAt(i); //真正删除的方法
                        return true;
                    }
                    if (++i == items.length)
                        i = 0;
                } while (i != putIndex);//一直不断的循环取出来做判断
            }
            return false;
        } finally {
            lock.unlock();
        }
    }

    /**
     * 是否包含一个元素
     */
    public boolean contains(Object o) {
        if (o == null) return false;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            if (count > 0) {
                final int putIndex = this.putIndex;
                int i = takeIndex;
                do {
                    if (o.equals(items[i]))
                        return true;
                    if (++i == items.length)
                        i = 0;
                } while (i != putIndex);
            }
            return false;
        } finally {
            lock.unlock();
        }
    }

    /**
     * 清空队列
     *
     */
    public void clear() {
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int k = count;
            if (k > 0) {
                final int putIndex = this.putIndex;
                int i = takeIndex;
                do {
                    items[i] = null;
                    if (++i == items.length)
                        i = 0;
                } while (i != putIndex);
                takeIndex = putIndex;
                count = 0;
                if (itrs != null)
                    itrs.queueIsEmpty();
                for (; k > 0 && lock.hasWaiters(notFull); k--)
                    notFull.signal();
            }
        } finally {
            lock.unlock();
        }
    }

    /**
     * 取出所有元素到集合
     */
    public int drainTo(Collection<? super E> c) {
        return drainTo(c, Integer.MAX_VALUE);
    }

    /**
     * 取出所有元素到集合
     */
    public int drainTo(Collection<? super E> c, int maxElements) {
        checkNotNull(c);
        if (c == this)
            throw new IllegalArgumentException();
        if (maxElements <= 0)
            return 0;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int n = Math.min(maxElements, count);
            int take = takeIndex;
            int i = 0;
            try {
                while (i < n) {
                    @SuppressWarnings("unchecked")
                    E x = (E) items[take];
                    c.add(x);
                    items[take] = null;
                    if (++take == items.length)
                        take = 0;
                    i++;
                }
                return n;
            } finally {
                // Restore invariants even if c.add() threw
                if (i > 0) {
                    count -= i;
                    takeIndex = take;
                    if (itrs != null) {
                        if (count == 0)
                            itrs.queueIsEmpty();
                        else if (i > take)
                            itrs.takeIndexWrapped();
                    }
                    for (; i > 0 && lock.hasWaiters(notFull); i--)
                        notFull.signal();
                }
            }
        } finally {
            lock.unlock();
        }
    }

}

三、LinkedBlockingQueue

接下来看看LinkedBlockingQueue的部分源码。

package java.util.concurrent;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import java.util.AbstractQueue;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.Consumer;

public class LinkedBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {
    private static final long serialVersionUID = -6903933977591709194L;

    /**
     * 链表节点类
     */
    static class Node<E> {
        E item;
        Node<E> next;//下一节点
        Node(E x) { item = x; }
    }

    /** 链表大小 ，默认大小 是Integer.MAX_VALUE */
    private final int capacity;

    /**当前队列中存放的元素个数，注意是原子类*/
    private final AtomicInteger count = new AtomicInteger();

    /**
     * 链表队列头节点
     */
    transient Node<E> head;

    /**
     * 链表队列尾节点
     */
    private transient Node<E> last;

    /** 取元素时的可重入锁 */
    private final ReentrantLock takeLock = new ReentrantLock();

    /**不为空条件*/
    private final Condition notEmpty = takeLock.newCondition();

    /**放元素是时的重入锁 */
    private final ReentrantLock putLock = new ReentrantLock();

    /** 不为满的条件 */
    private final Condition notFull = putLock.newCondition();

    /**
     * 不为空通知方法
     */
    private void signalNotEmpty() {
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();
        try {
            notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
    }

    /**
     * 不为满通知方法
     */
    private void signalNotFull() {
        final ReentrantLock putLock = this.putLock;
        putLock.lock();
        try {
            notFull.signal();
        } finally {
            putLock.unlock();
        }
    }

    /**
     * 进队
     *
     * @param node the node
     */
    private void enqueue(Node<E> node) {
        last = last.next = node;
    }

    /**
     * 出队
     */
    private E dequeue() {
        Node<E> h = head;
        Node<E> first = h.next;
        h.next = h; // help GC
        head = first;
        E x = first.item;
        first.item = null;
        return x;
    }

    /**
     * 取和入都上锁，此时无法取和放
     */
    void fullyLock() {
        putLock.lock();
        takeLock.lock();
    }

    /**
     * 释放锁
     */
    void fullyUnlock() {
        takeLock.unlock();
        putLock.unlock();
    }

    /**
     * 构造函数
     */
    public LinkedBlockingQueue() {
        this(Integer.MAX_VALUE);
    }

    /**
     * 构造函数
     *
     */
    public LinkedBlockingQueue(int capacity) {
        if (capacity <= 0) throw new IllegalArgumentException();
        this.capacity = capacity;
        last = head = new Node<E>(null);
    }

    /**
     * 构造函数
     */
    public LinkedBlockingQueue(Collection<? extends E> c) {
        this(Integer.MAX_VALUE);
        final ReentrantLock putLock = this.putLock;
        putLock.lock(); //取得放入锁
        try {
            int n = 0;
            for (E e : c) {
                if (e == null)
                    throw new NullPointerException();
                if (n == capacity)
                    throw new IllegalStateException("Queue full");
                enqueue(new Node<E>(e));
                ++n;
            }
            count.set(n);
        } finally {
            putLock.unlock();
        }
    }

     //阻塞等待放入
    public void put(E e) throws InterruptedException {
        if (e == null) throw new NullPointerException();
        int c = -1;
        Node<E> node = new Node<E>(e);
        final ReentrantLock putLock = this.putLock;
        final AtomicInteger count = this.count;
        putLock.lockInterruptibly(); //取得放入锁
        try {
            while (count.get() == capacity) {//队列已满
                notFull.await();
            }
            enqueue(node);//入队
            c = count.getAndIncrement();//当前队列中元素个数加1
            if (c + 1 < capacity)
                notFull.signal();
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
    }

    /**
     *带超时时间的阻塞等待放入，队列不满。放入成功返回true，否则返回fasle
     */
    public boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException {

        if (e == null) throw new NullPointerException();
        long nanos = unit.toNanos(timeout);
        int c = -1;
        final ReentrantLock putLock = this.putLock;
        final AtomicInteger count = this.count;
        putLock.lockInterruptibly();
        try {
            while (count.get() == capacity) {
                if (nanos <= 0)
                    return false;
                nanos = notFull.awaitNanos(nanos);
            }
            enqueue(new Node<E>(e));
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                notFull.signal();
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
        return true;
    }

    /**
     * 非阻塞放入。队列不满放入成功返回true，否则返回fasle
     */
    public boolean offer(E e) {
        if (e == null) throw new NullPointerException();
        final AtomicInteger count = this.count;
        if (count.get() == capacity)
            return false;
        int c = -1;
        Node<E> node = new Node<E>(e);
        final ReentrantLock putLock = this.putLock;
        putLock.lock();
        try {
            if (count.get() < capacity) {
                enqueue(node);
                c = count.getAndIncrement();
                if (c + 1 < capacity)
                    notFull.signal();
            }
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
        return c >= 0;
    }
    //阻塞等待取出元素
    public E take() throws InterruptedException {
        E x;
        int c = -1;
        final AtomicInteger count = this.count;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lockInterruptibly();
        try {
            while (count.get() == 0) {
                notEmpty.await();
            }
            x = dequeue();
            c = count.getAndDecrement();
            if (c > 1)
                notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
            signalNotFull();
        return x;
    }
    //带有超时时间等待的取出元素
    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        E x = null;
        int c = -1;
        long nanos = unit.toNanos(timeout);
        final AtomicInteger count = this.count;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lockInterruptibly();//等待时可抛出异常跳出
        try {
            while (count.get() == 0) {
                if (nanos <= 0)
                    return null;
                nanos = notEmpty.awaitNanos(nanos);//超时等待
            }
            x = dequeue();
            c = count.getAndDecrement();
            if (c > 1)
                notEmpty.signal();//不这空条件成立
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
            signalNotFull();
        return x;
    }
    //取队头元素。没有的话返回null，有的话返回元素，并将队列中删除此元素
    public E poll() {
        final AtomicInteger count = this.count;
        if (count.get() == 0)
            return null;
        E x = null;
        int c = -1;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();//获得取得锁
        try {
            if (count.get() > 0) {
                x = dequeue();//出队
                c = count.getAndDecrement();//当前队列中元素个数减去1
                if (c > 1)
                    notEmpty.signal();//不为空条件成功
            }
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
            signalNotFull();
        return x;
    }
    //取队头元素，但不从队列中删除 ，没有的话返回null,不阻塞
    public E peek() {
        if (count.get() == 0)
            return null;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();//获得取得锁
        try {
            Node<E> first = head.next;
            if (first == null)
                return null;
            else
                return first.item;
        } finally {
            takeLock.unlock();
        }
    }

    /**
     * 删除时要同时取得放入锁和取得锁
     */
    public boolean remove(Object o) {
        if (o == null) return false;
        fullyLock();//同时取得放入锁和取得锁
        try {
            for (Node<E> trail = head, p = trail.next;
                 p != null;
                 trail = p, p = p.next) {
                if (o.equals(p.item)) {
                    unlink(p, trail);
                    return true;
                }
            }
            return false;
        } finally {
            fullyUnlock();
        }
    }

    /**
     * 是否包含
     */
    public boolean contains(Object o) {
        if (o == null) return false;
        fullyLock();//同时取得放入锁和取得锁
        try {
            for (Node<E> p = head.next; p != null; p = p.next)
                if (o.equals(p.item))
                    return true;
            return false;
        } finally {
            fullyUnlock();
        }
    }
}

从LinkedBlockingQueue的源码中，我们可以看出他和ArrayBlockingQueue主要有以下两点区别：

1、ArrayBlockingQueue数据是放在一个数组中。LinkedBlockingQueue是放在一个Node节点中，构成一个链接。

2、ArrayBlockingQueue取元素和放元素都是同一个锁，而LinkedBlockingQueue有两个锁，一个放入锁，一个取得锁。分别对应放入元素和取得元素时的操作。这是由链表的结构所确定的。但是删除一个元素时，要同时获得放入锁和取得锁。

四、SynchronousQueue

SynchronousQueue 这个队列实现了 BlockingQueue接口。该队列的特点
1.容量为0，无论何时 size方法总是返回0
2. put操作阻塞， 直到另外一个线程取走队列的元素。
3.take操作阻塞，直到另外的线程put某个元素到队列中。
4. 任何线程只能取得其他线程put进去的元素，而不会取到自己put进去的元素

public SynchronousQueue(boolean fair) {
        transferer = fair ? new TransferQueue() : new TransferStack();
    }

构造方法上接收boolean参数，表示这是一个公平的基于队列的排队模式，还是一个非公平的基于栈的排队模式。